Gas tight polypropylene peel film

ABSTRACT

The present invention is related to a coextruded multilayer polyolefin film being heat sealable and peelable onto polypropylene based substrates of at least 3 layers, comprising at least a peelable top layer, at least an intermediary layer and at least a base layer, characterised in that at least one of the intermediate layers comprises an easy lowable low melting polymer with a melt flow index at least equal or higher than the melt flow index of the base layer, said easy flowable polymer representing at least 60% by weight of the total weight of said multilayer polymer film.

FIELD OF THE INVENTION

The present invention is related to a transparent coextruded gas tight peelable seal polyolefin film for packaging applications, and in particular for various types of standing pouches.

STATE OF THE ART

In gas-tight food packages different mixtures of gases are added to the content in order to retain the filled product properties such as aromas or freshness during the storage. This implies that the gas has to be kept in the package during the shelf life of the product. The packaging film or laminate constituting the package is therefore required to have low gas and aroma diffusion rates. Barrier properties of individual polymeric packaging materials and different product constructions thereof for various sorts of aromas and gasses, including water vapour, O₂, N₂, CO₂, are known in the prior art.

On the other hand the package should be produced in such a way that it is physically fully closed and that no leakages occur. Absence of leakages is therefore an essential characteristic for gas tight packaging applications. In packaging designs mainly the sealing areas are critical. If the seams are physically unsufficiently joined in the sealing area, the packages will have a high risk of leakages.

If a package in addition to be the gas-tight sealed should also allow an easier opening, the sealing film needs to have peelable characteristics. This implies that the seal strength of the seams cannot be infinite, but should be controllable in such a way that the film is of an easy opening type (e.g. opening without the use of scissors) Peelability and gas-tightness are characteristics that are difficult to match.

The U.S. Pat. No. 5,997,968 discloses a coextruded multilayer polyolefin film for lidding applications comprising an opaque base layer an intermediate layer, and a peelable layer of two incompatible polyolefins. Unfortunately, only one of both polyolefins are disclosed as a polypropylene, the other polymer is defined by its incompatibility with polypropylene and its melting range between 105 and 140° C. This large melting range and the fact that the polymer should be incompatible to the former does no give any indication about the nature of the second polyolefin.

This document further teaches the use of a thin intermediate layer of less than 12 μm to avoid delamination and residues during the opening of the peel layer. These specific problems are not observed by the inventor in particular for non-oriented cast or blown coextruded films. Moreover the intermediate layer of 12 μm is too thin to represent a solution for gas tight and zero leakage rate packaging.

The documents U.S. Pat. No. 4,666,778 presents a laminar ethylenic peel layer based on a majority of ethylene polymers or copolymers. These films are not coextruded and have no peel properties on polypropylene based substrates.

Johnson in U.S. Pat. No. 5,358,791 discloses a sterilisable multilayer film with two outer layers and a core layer. In this document, the sterilisability is reached by the use of high temperature resistant polymers such as polypropylene and high-density polyethylene. This is totally antagonistic to a request of low-temperature seal or easy flowability needed for gas-tight packaging. Low temperature seal polymers with melting points lower than 135° C. exclude the use of HDPE. Furthermore, the weight proportion of the low melting polypropylene should be at least 50%. On the other hand, the addition of HDPE negatively influences the transparency of the films and would produce additional problems between the peelable layer and the outer layer, if the adhesion between the individual layers become too low, a significant risk of layer delamination during opening of the film exists, especially if the adhesion forces are lower than the peel forces.

Kong discloses in U.S. patent application 2001/00 31371-A1 a sealable film with a separable layer. The inner and the sealing layer are basically high melting and high viscosity polymers such as isotactic pp homopolymers or propylene-ethylene bloc copolymers. In this document, the inner layer is described as being polypropylene, high-density polyethylene and ethylene-propylene bloc copolymers. All of them are not low melting and easy flowable materials. This document furthermore precises that the inner layer comprises more than 80% isotactic polypropylene, the low melting polymers used in this case are only present in the sealable layer, which is on top of the separable layer.

For those skilled in the art of peelable structures, it is evident that sealable materials on top of the peelable layer, also called the break-away peel, should be as thin as possible. This kind of sealable layer is broken away before the peel can be opened.

In the U.S. Pat. No. 5,773,136, Alder discloses a multilayer film, wherein the peeling occurs in the intermediate core layer. This OPP film does not contain enough sealable mass because of their limited thickness, to have a tight seal in the faults of e.g a standing pouch. Furthermore, the individual thickness of the base layer is not mentioned in this document, only percentages of the individual layers are given.

AIMS OF THE INVENTION

The present invention aims to provide a transparent peelable seal film presenting zero leakage properties within a package by introducing an easy flowable intermediary layer able to fill all gaps or voids created within the folds in the sealing area of a package.

SUMMARY OF THE INVENTION

The present invention provides a coextruded multilayer polyolefin film of at least three layers, being heat sealable and peelable onto polypropylene based substrates, comprising at least a peelable top layer, at least an intermediary layer and at least a base layer, characterised in that at least one of the intermediate layers comprises an easy flowable low melting polymer with a melt flow index at least equal or higher than the melt flow index of the base layer.

In a first embodiment of the present invention the coextruded multilayer film is characterised in that said easy flowable low melting polymer of at least one of the intermediary layers is selected from the group consisting of copolymers comprising propylene and ethylene, propylene and butylene, propylene and any other olefin having 5-10 carbon atoms.

In a second embodiment of the present invention the coextruded multilayer film is characterised in that said easy flowable low melting polymer of at least one of the intermediary layers is selected from the group consisting of polypropylene terpolymers containing ethylene, propylene, butylene or any other olefin having 5-10 carbon atom.

In an additional embodiment of the present invention the coextruded multilayer film is characterised in that said easy flowable low melting polymer of at least one of the intermediary layers is selected from the group consisting of syndiotactic polypropylenes.

In a further embodiment of the present invention a coextruded multilayer film is characterised in that said easy flowable low melting polymer of at least one of the intermediary layers is selected from the group consisting of mixtures of the above said polymers with poly 1-butene homopolymers or poly 1-butene ethylene copolymers and/or homogeneous and heterogeneous ethylene alpha-copolymers with a density below 0.920 g/cm³.

Another key aspect of the present invention is that said base layer is selected from the group consisting of propylene homopolymers and copolymers of propylene and ethylene, propylene and butylene, propylene and any other olefin having 5-10 carbon atoms, or polypropylene terpolymers containing ethylene, propylene, butylene or any other olefin having 5-10 carbon atoms

An additional key aspect of the present invention is that the easy flowable low melting polymer represents at least 60% by weight of the total weight of the multilayer polymer film and that said film has a thickness of at least 15 μm.

The present invention discloses also the use of a coextruded multilayer heat sealable and peelable polymer film in combination with diffusion tight films for gas tight packagings and for various types of gas tight standing pouches.

SHORT DESCRIPTION OF THE DRAWINGS

FIG. 1 represents the peel seal curve against itself versus the sealing temperature in a standard version (prior art) and according to the present invention (gas tight).

FIG. 2 represents the peel seal curve against BOPP versus the sealing temperature in a standard version (prior art) and according to the present invention (gas tight).

FIG. 3 represents the result of a leakage test in a comparison between the conventional peel film containing a PP homopolymer (PPh) core layer and the gas tight films containing an intermediate core layer with ethylene-butylene PP terpolymer and blends of ethylene-butylene PP terpolymer and homogeneous LLDPE (p=0.902 g/cm³) in a ratio 90/10 (PPter1), 75/25 (PPter2), 60/40 (PPter3) and 40/60 (PPter2).

DETAILED DESCRIPTION OF THE INVENTION

In the present invention zero leakages are provided by an intermediate coextruded layer between the peelable layer and the base layer. The intermediate layer contains a sufficient amount of easy-flowable low melting polymer. The thickness of such a film reaches approximately 40 μm and the seal/peal layer has only a thickness of approximately 5-8 μm. This means that the intermediate layer has a thickness of approximately 30 μm and the base layer a thickness of approximately 10 μm.

The intermediate layer in the present invention contains low melting PP polymers such as

-   -   copolymers of propylene and ethylene, propylene and butylene,         propylene and any other olefin having 5-10 carbon atoms, or     -   polypropylene terpolymers containing ethylene, propylene,         butylene or any other olefin having 5-10 carbon atoms, or     -   syndiotactic polypropylene or     -   mixtures of the above said polymers with poly 1-butene and/or         homogeneous and heterogeneous branched ethylene alpha-copolymers         with a density below 0.920 g/cm³.

The homogeneous and heterogeneous branched ethylene alpha-copolymers with a density below 0.920 g/cm³ should be selected as such that they are sufficiently compatible with the former in order to avoid a distortion of the film optics. Moreover a melt-index of the intermediate layer at least equal or higher than the one of the base layer helps to facilitate the flow during sealing. The melt indexes mentioned in this invention are measured according to ASTM D1238 at a temperature of 190° C. and with a load of 2,16 KS.

DESCRIPTION OF A PREFERRED EMBODIMENT OF THE INVENTION

An 19 μm OPP laminate containing e.g. a 40 μm coextruded film with a 8 ∥m peelable PP layer based on e.g. Novolen VP 9201 (MI=6, BASF) and a 14 μm homo PP core layer and 18 μm homo PP base layer containing respectively Moplen X30S (MI=8.5, ρ=0.905 g/cm³, Basell) and Stamylan P 16B10FC (MI=5.5, ρ=0.905 g/cm³, DSM). This construction does not provide a gas-tight package.

A 19 μm OPP laminate however with a 6 μm peelable layer and a 26 μm core layer consisting of 60% Adsyl 5X37F (MI=7, ρ=0.905 g/cm³, Basell) and 40% Exact 0203 (MI=3, ρ=0.902 g/cm³, Dex plastomers) combined with a base layer of 8 μm Moplen C30S (MI=6, ρ=0.905, Basell) will provide gas-tight packages.

A 39 μm OPP laminate with a 4 μm peelable layer, a 32 μm core layer containing 80% Adayl SX37F (MI=7, ρ=0.905 g/cm³, Basell) and 25% PB8220 (MI=2, ρ=0.897/cm³, Basell) and 4 μm Stamylan P 16R10PC (MI=5.5, ρ=0.905 g/cm³, DSM) as base layer will also result in a gas-tight package.

Experimental Sealing Conditions

Prior to sealing the 40 μm peel seal films were laminated against a 20 μm standard coextruded BOPP film. Peel seal curves were performed on a Kopp sealer. Peelability against itself was evaluated with a pressure of 1.3 bar on the two heated jaws. The sealing dwell time corresponded with 0.5 sec. Peelability onto OPD was evaluated in an A to B sealing with the upper jaw heated and the lower containing a 60 shore A rubber at room temperature. The applied pressure corresponded with 1.3 bar applied during 1 sec.

Leakage Detection

In order to identify leakages small sachets were sealed according to the above sealing conditions. Prior to sealing a rhodamine solution was added to the sachets. The seals were considered as closed if no single visible migration of rhodamine was observed from the inside to the outside of the sacchets (pass-fail test). An alternative test is the under-water Dinkelberg leakage tester with an applied under-pressure (300-760 mbar) selected as such that it does not peel open the package. 

1-10. (canceled)
 11. A coextruded multilayer polyolefin film being heat sealable and peelable onto polypropylene based substrates, the film comprising a peelable top layer, a base layer, and an intermediate layer comprising an easy flowable low melting polymer with a melt flow index about equal to or higher than the melt flow index of the base layer, wherein the easy flowable polymer represents at least 60% by weight of the total weight of the multilayer polyolefin film.
 12. The coextruded multilayer film of claim 11, wherein the easy flowable low melting polymer is a copolymer selected from the group consisting of a copolymer comprising propylene and ethylene, a copolymer comprising propylene and butylene, and a copolymer comprising propylene and an olefin having 5-10 carbon atoms.
 13. The coextruded multilayer film of claim 11, wherein the easy flowable low melting polymer is a terpolymer comprising polypropylene and two polymers selected from the group consisting of ethylene, butylene and an olefin having 5-10 carbon atoms.
 14. The coextruded multilayer film of claim 11, wherein the easy flowable low melting polymer is a syndiotactic polypropylene.
 15. The coextruded multilayer film of claim 11, wherein the easy flowable low melting polymer comprises a mixture of (a) a copolymer selected from the group consisting of a copolymer comprising propylene and ethylene, a copolymer comprising propylene and butylene, a copolymer comprising propylene and an olefin having 5-10 carbon atoms, and a terpolymer comprising polypropylene and two polymers selected from the group consisting of ethylene, butylene or an olefin having 5-10 carbon atoms, and (b) a polymer selected from the group consisting of a poly 1-butene hompolymer, a poly 1-butene ethylene copolymer, a homogenous ethylene alpha-copolymer with a density below 0.920 g/cm³, and heterogeneous ethylene alpha-copolymer with a density below 0.920 g/cm³.
 16. The coextruded multilayer film of claim 11, wherein the said base layer is selected from the group consisting of a propylene homopolymer, a copolymer of propylene and ethylene, a copolymer of propylene and butylene, and a copolymer of propylene and an olefin having 5-10 carbon atoms, and a terpolymer comprising polypropylene and two polymers selected from the group consisting of ethylene, butylene and an olefin having 5-10 carbon atoms.
 17. The coextruded multilayer film of claim 11, wherein the easy flowable layer has a thickness of at least 15 μm.
 18. A gas tight package comprising the multilayer film of claim
 11. 19. A gas tight package comprising the multilayer film of claim
 12. 20. A gas tight package comprising the multilayer film of claim
 13. 21. A gas tight package comprising the multilayer film of claim
 14. 22. A gas tight package comprising the multilayer film of claim
 15. 23. A gas tight package comprising the multilayer film of claim
 16. 24. A gas tight package comprising the multilayer film of claim
 17. 25. A gas tight package comprising the film of claim 11 in combination with a diffusion tight film.
 26. The gas tight package of claim 18 wherein the package is a standing pouch.
 27. The gas tight package of claim 25 wherein the package is a standing pouch.
 28. A polymer film that is heat-sealable onto a polypropylene based substrate, the film comprising a polyolefin top layer that is peelable onto the substrate, a polyolefin base layer, and an intermediate layer comprising an easy flowable low melting polyolefin having a melt flow index about equal to or higher than the melt flow index of the base layer, wherein the easy flowable polyolefin represents at least 60% by weight of the total weight of the polymer film.
 29. The film of claim 28, wherein the intermediate layer comprises a copolymer selected from the group consisting of a copolymer comprising propylene and ethylene, a copolymer comprising propylene and a butylene, and a copolymer comprising propylene and an olefin having 5-10 carbon atoms.
 30. The film of claim 28, wherein the wherein the intermediate layer comprises a polymer comprising polypropylene and two polymers selected from the group consisting of ethylene, butylene and an olefin having 5-10 carbon atoms.
 31. The film of claim 28, wherein the wherein the intermediate layer comprises a syndiotactic polypropylene.
 32. The film of claim 28, wherein the wherein the intermediate layer comprises a mixture of (a) a copolymer selected from the group consisting of a copolymer comprising propylene and ethylene, a copolymer comprising propylene and butylene, a copolymer comprising propylene and an olefin having 5-10 carbon atoms, and a polymer comprising polypropylene and two polymers selected from the group consisting of ethylene, butylene and an olefin having 5-10 carbon atoms, and (b) a polymer selected from the group consisting of a poly 1-butene hompolymer, a poly 1-butene ethylene copolymer, a homogenous ethylene alpha-copolymer with a density below 0.920 g/cm³, and heterogeneous ethylene alpha-copolymer with a density below 0.920 g/cm³.
 33. The film of claim 28, wherein the said base layer is selected from the group consisting of a propylene homopolymer, a copolymer of propylene and ethylene, a copolymer of propylene and butylene, and a copolymer of propylene and an olefin having 5-10 carbon atoms, and a polymer comprising polypropylene and two polymers selected from the group consisting of ethylene, butylene or an olefin having 5-10 carbon atoms.
 34. The film of claim 28, wherein the easy flowable layer has a thickness of at least 15 μm.
 35. A gas tight package comprising the multilayer film of claim
 28. 36. A gas tight package comprising the film of claim 28 in combination with a diffusion tight film.
 37. The gas tight package of claim 35 wherein the package is a standing pouch. 